When incorporating alloplastic implants into organs, an exact fit is essential in order to ensure good retention and permanent support for the physiological stresses acting upon the underlying tissue, and to evenly distribute the forces acting upon the implant itself. The above is applicable, for example, to enossal and alveolar implants in the jaw, exoprosthetic contact lenses, hip joint prostheses, and also to the common dental alloplastic implants such as fillings, crowns and bridges. For enossal implants or contact lenses, the currently applied prosthetic devices are normally chosen from a kit containing various sizes, or else they are fitted to the organ after the organ has been trimmed to a standard shape.
In restorative dentistry, the established technique consists of the use of a custom-shaped implant such as can be produced by the conventional laboratory procedures. For the following description of the novel method, the dental scenario has been chosen by way of example.
Any kind of dental implant whose purpose is the permanent restoration of the tooth, particularly the crown of the tooth, to regain original appearance and function must be considered as alloplastic (i.e. a foreign body). The familiar techniques utilize fillings fabricated from precious cast metals, amalgam, ceramics or dental composite materials.
Conventional fillings in the form of inlays, onlays or overlays yield satisfactory physical properties and morphology, but a negative aspect is the generally high cost, especially when precious metals are used. The classic inlay fabrication, whether based on metal or ceramic filling materials, requires a string of time-consuming procedures such as: mold casting, model preparation, wax modeling, embedding of the wax model, die-casting, injection molding or stuffing, extrusion and high-temperature curing.
U.S. Pat. No. 4,182,312 teaches the reading of three-dimensional contour data of teeth and surrounding tissues directly inside the patient's mouth by means of a mechanical pantograph, in order to control a tool to fabricate the prosthesis, similar to a copy-milling machine. In order to read the contour data, a probe stylus rigidly connected to the pantograph must be manually guided along the contour of the tooth or gums. This procedure seems impractical, since a large number of translatory sweeps is required to faithfully survey the object under investigation, resulting in time consumption and discomfort to the patient. Another complication results from the need to compensate for the finite dimensions of the probe tip.
Furthermore, the need to establish a reference coordinate system results in temporarily fastening a tray to the patient's jaw, causing not only additional discomfort, but also restricting maneuverability of the probe. From a practitioner's point of view, these auxiliary devices and restrictions seem to be more cumbersome than the standard prosthetic methods involving the use of casts.
Besides the mechanical contour measurement taught in above-mentioned U.S. Pat. No. 4,182,312, several optical three-dimensional recording devices have been described in various publications including G. W. Butcher et al, "The Reflex Optical Plotter", Brit. Dent. J., 1981, 151, p. 304; E. M. Mikhail, Chapters 17-19 in "Photogrammetry", pp. 579-582, ed. F. H. Moffitt, Harper & Row, New York, 1980; and K. Takasaki, "Moire Topography, Systems and Applications", Chapter 8 in "Handbook of Non-Topographic Photogrammetry", ed. H. M. Karara, Am. Soc. of Photogrammetry, Everybody Press, 1979. All of them, however, serve diagnostic purposes exclusively. The plotters used by Butcher and Mikhail require time-consuming operator assistance. Moire Topography is a method for automatic contour mapping. It has, however, so far been limited (Takasaki 1979) to a coarse, nonabsolute depth measurement.